Added integration support for DataDog APM Tracing

vendor/gopkg.in/DataDog/dd-trace-go.v1/ddtrace/tracer/sampler.go

@@ -0,0 +1,72 @@
+package tracer
+
+import (
+	"math"
+	"sync"
+
+	"gopkg.in/DataDog/dd-trace-go.v1/ddtrace"
+)
+
+// Sampler is the generic interface of any sampler. It must be safe for concurrent use.
+type Sampler interface {
+	// Sample returns true if the given span should be sampled.
+	Sample(span Span) bool
+}
+
+// RateSampler is a sampler implementation which randomly selects spans using a
+// provided rate. For example, a rate of 0.75 will permit 75% of the spans.
+// RateSampler implementations should be safe for concurrent use.
+type RateSampler interface {
+	Sampler
+
+	// Rate returns the current sample rate.
+	Rate() float64
+
+	// SetRate sets a new sample rate.
+	SetRate(rate float64)
+}
+
+// rateSampler samples from a sample rate.
+type rateSampler struct {
+	sync.RWMutex
+	rate float64
+}
+
+// NewAllSampler is a short-hand for NewRateSampler(1). It is all-permissive.
+func NewAllSampler() RateSampler { return NewRateSampler(1) }
+
+// NewRateSampler returns an initialized RateSampler with a given sample rate.
+func NewRateSampler(rate float64) RateSampler {
+	return &rateSampler{rate: rate}
+}
+
+// Rate returns the current rate of the sampler.
+func (r *rateSampler) Rate() float64 {
+	r.RLock()
+	defer r.RUnlock()
+	return r.rate
+}
+
+// SetRate sets a new sampling rate.
+func (r *rateSampler) SetRate(rate float64) {
+	r.Lock()
+	r.rate = rate
+	r.Unlock()
+}
+
+// constants used for the Knuth hashing, same as agent.
+const knuthFactor = uint64(1111111111111111111)
+
+// Sample returns true if the given span should be sampled.
+func (r *rateSampler) Sample(spn ddtrace.Span) bool {
+	s, ok := spn.(*span)
+	if !ok {
+		return false
+	}
+	r.RLock()
+	defer r.RUnlock()
+	if r.rate < 1 {
+		return s.TraceID*knuthFactor < uint64(r.rate*math.MaxUint64)
+	}
+	return true
+}